1. Field of the Invention
The present invention relates to an audio amplifier using a pulse width modulation, and more particularly, to a pulse width modulation type audio amplifier which average switching frequency or a cutoff frequency of a low pass filter is variable in accordance with frequency bandwidth of input signal.
2. Description of the Related Art
FIG. 1 is a block diagram showing a conventional pulse width modulation (PWM) audio amplifier.
As shown in FIG. 1, the conventional pulse width modulation type audio amplifier basically includes a pulse width modulator 1, a power switching amplifier 2, and a low pass filter 3.
The pulse width modulator 1 quantizes a signal inputted from external device and converts the signal into a pulse signal with a selective value among 0 (zero) and 1 (one). At that time, width of the outputted signal is determined by magnitude of the inputted signal. In other words, at time rate which each pulse to be outputted maintains 0 (zero) and 1 (one) in accordance with the magnitude of the inputted signal is continuously varied. Meanwhile, the average switching frequency for each pulse is fixed to a specific value.
Meanwhile, the power switching amplifier 2 amplifies magnitude of a pulse width modulated small signal pulse having information about the inputted signal. Finally, the low pass filter 3 converts the pulse signal amplified by the power switching amplifier 2 into a continuous time signal and transfers the continuous time signal to a speaker.
In the conventional pulse width modulation type audio amplifier, since the output signal is switched at a high speed and amplitude of the pulse is relatively large, a spike type pulse is successively occurred, so that electromagnetic interference (EMI) is occurred on the peripheral equipments. Moreover, the higher the average switching frequency of the pulse width modulation type audio amplifier is, the greater electric power consumed in the switching operation, power efficiency of the amplifier is deteriorated. Since the average switching frequency is always fixed to a high frequency, these drawbacks are more remarkable when the fixed high frequency is applied to even an input signal of low bandwidth of frequency.